2. FIELD OF THE INVENTION
This invention relates to a method of preparing very low pour point oils suitable for use as transformer oils. More particularly, this invention relates to an improved process for producing low pour point transformer oils from paraffinic crudes.
2. DESCRIPTION OF THE PRIOR ART
Transformer oils are known in the prior art as high stability electrical insulating oils and are also used in other electrical equipment such as circuit breakers. In addition to possessing relatively low viscosity, high dielectric strength and a relatively high flash point, these oils are further characterized in that they must have a relatively low pour point. This is particularly necessary where the oils are to be used in colder climates. Additionally, these oils must be low in corrosive agents such as acid, alkali and sulfur and resistant to oxidation and sludge formation.
Several methods for preparing insulating oils are known in the prior art. In general, they are produced from wax-free naphthenic crude oils which are not native to many parts of the world and consequently command premium prices and involve high transportation costs. Although these crudes permit production of exceptionally low pour point insulating oils without the need for dewaxing or special attention to the degree of fractionation or distillate cut width, they also contain high percentages of sulfur and nitrogen which must be removed in order to satisfy the stringent stability requirements of insulating oils.
Extremely stable insulating oils produced either totally or partially from paraffinic crudes by conventional dewaxing techniques are also used in certain applications where moderate climatic conditions do not demand oils with especially low cloud or pour points. When exceptionally low pour points are required, however, deep dewaxing of paraffinic distillates at temperatures below -40.degree. F cannot compete economically with the manufacture of these oils from naphthenic crudes.
A process which avoids deep dewaxing and produces competitively priced, low viscosity oils with exceptionally low pour points from paraffinic distillates was disclosed in U.S. Pat. No. 2,906,688. In this process a broad, wax-containing fraction is first sharply fractionated to yield a narrow heart cut of suitable viscosity which is then dewaxed at about 0.degree. F to yield a dewaxed oil having a pour point of -50.degree. F, or lower. The narrow cut also may be solvent extracted prior to dewaxing without materially affecting the pour point of the product. The difficulty associated with this process is related to the nature of the low pour point oils produced. Although they have low pour points, their cloud points, which mark the onset of wax precipitation, are relatively high, approximating the dewaxing temperature used in making them. This, in turn, makes the pour point of the product extremely sensitive to waxy contaminants such as paraffin wax, which would certainly be encountered in process lines comprising solvent treating (extraction), hydrofining and dewaxing units which normally operate on relatively high pour point, wax-bearing streams. In fact, it has been found that the addition of as little as 0.5 percent of a paraffin wax to an insulating oil prepared by the aforedescribed process and having a cloud point of -12.degree. F will raise the pour point from -50.degree. F to -5.degree. F, whereas the addition of the same amount of paraffin wax to a wax-free, naphthenic oil having the same viscosity and pour point does not affect the pour point at all.
U.S. Pat. No. 3,627,673 discloses a process for producing low pour point transformer oils from paraffinic crudes which eliminates the highly specialized fractionating tower needed for the initial narrow cut distillate required in U.S. Pat. No. 2,906,688 as well as avoiding the waxy contaminant sensitivity of the product produced by said process. The U.S. Pat. No. 3,627,673 process comprises taking a narrow cut having a 5 to 95 LV (liquid volume) % boiling range between 550.degree. to 750.degree. F from a conventional crude oil vacuum pipe still, solvent extracting, dewaxing and hydrofining the dewaxed raffinate, followed by fractionally distilling the hydrofined dewaxed raffinate to obtain a narrow cut (heart cut) having a 5/95 LV% boiling range of from 580.degree. to 720.degree. F or narrower. However, the fractionating or rerun column required in this process is rather complex and contains a relatively large number of plates therein.
It would be a great improvement to the art if one could obtain low pour point transformer oils from paraffinic crudes without encountering the waxy contaminant sensitivity of the product produced by the process in U.S. Pat. No.2,906,688 and at the same time avoid the necessity of the special fractionating columns required in both the U.S. Pat. Nos. 2,906,688 and 3,627,673 processes.